ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 187 



by huge cytoclasts or pliag. cytes. They are unusually large, and have 

 very large nuclei. The presence of abortive villi on the surface of the 

 chorion in the vicinity of the hint of a girdle is a further proof of its 

 suggested presence. The data seem to show that the primitive type of 

 placentation was more diffuse than in existing myomorph rodents, and 

 throw light on the derivation of discoidal from zonary placentation. 



Development of Blood-vascular System of the Chick.* — Dr. N. 

 Uskow is of opinion that the hen's egg is, in the true sense of the word, 

 a giant-cell. Its jjrotoplasm is ordinarily so thickly impregnated with 

 the fine elements of the white yolk that its presence is only detectable 

 on the appearance of nuclei ; the elements of the yellow yolk are at first 

 apparently much more closely connected with the protoplasm ; in later 

 stages of development the protoplasm may be seen even though there 

 are no nuclei ; but the presence of the latter may serve as a proof of 

 the protoplasmic nature of the visible network. All the changes in 

 the egg during the development of the embryo may be regarded as the 

 gradual agglomeration of the protoplasm towards the periphery, and 

 as its segmentation with formation of numerous nuclei, and later, cells. 

 These two processes succeed one another, and begin at the upper end of 

 the vertical axis of the egg. The process of segmentation does not go 

 on uninterruptedly, for there are some breaks after the appearance of 

 cells in the peripheral portions. 



The hypoblast is differently formed in different regions, and may be 

 conveniently divided into three parts : — (1) Marginal portion of the 

 hypoblast, where the protoplasm contains nuclei, and has no signs of 

 cell-formation, save a fueble indication of this process in the upper 

 layers; (2) Intermediate- portion, with cylindrical, not fully developed 

 cells ; and (3) Central part, with distinct epithelial cells. It is to the 

 first two of these portions that the blood-vascular system owes its origin, 

 while the marginal portion also gives rise to the peripheral part of the 

 mesohlast. To explain their origin it is not necessary to make use of 

 the hypothesis of the emigration of cells through the yolk; all the 

 observed phenomena may be explained by supposing the elements to 

 arise at the place where they are found. 



Remak's cord is only one of the stages of a common form of develop- 

 ment of the vascular system, and is not the only one or the most 

 primitive ; the vessels and blood are developed below the mesoblast, 

 and are only later surrounded by it. The blood is not formed before 

 the vessels, or the vessels before the blood ; "as one process conditions 

 the appearance of the other, the two are simultaneous." The vascular 

 lumen is neither an intracellular nor an intercellular space. There is 

 no ground for regarding the first vessels which appear in the central 

 part of the zona pellucida and in the embryo as secondary. Some of 

 the vessels appear in the course of development as ve?icles. 



The author concludes that the formation of the blood and the vessels 

 of a chick may be taken as a distinct support of the view that the living 

 protoplasm of a fertilized ovum contains in different parts various 

 definite tissues of the organism from which it is derived. 



Development of Emu.j — Mr. W. A. Haswell gives an account of 

 his observations on the early stages in the development of the emu, 



* Mem. Acad. Imp. St.-Petcrsbourp, xxxv. (18S7) 48 pp. (2 pis.). 

 t Proc. Linn. Soc. N. S. Wales, i. (1887) pp. 577-600 (8 pis.). 



